Primary alcohols are a product class of compounds having a variety of industrial applications, this include a variety of biofuels and specialty chemicals. Primary alcohols also can be used to make a large number of additional industrial products including polymers and surfactants. Higher primary alcohols, also known as fatty alcohols, and their derivatives have numerous commercial applications, including use as surfactants, lubricants, plasticizers, solvents, emulsifiers, emollients, thickeners, flavors, fragrances, and fuels. Fatty alcohols can further be dehydrated to alpha-olefins, which have utility in the manufacture of polymers, lubricants, surfactants, plasticizers, and can also be used in fuel formulations.
Current technologies for producing fatty alcohols involve inorganic catalyst-mediated reduction of fatty acids to the corresponding primary alcohols. The fatty acids used in this process are derived from natural sources, e.g., plant and animal oils and fats, primarily coconut, palm, palm kernel, tallow and lard. These various sources have different fatty acid compositions; of particular importance are the varying acyl chain lengths that are present. As a consequence, the fatty alcohols derived from these fatty acids also have varying chain lengths. The chain length of fatty alcohols greatly impacts the chemical and physical properties of the molecules, and thus different chain lengths are used for different applications. Fatty alcohols are currently produced from, for example, hydrogenation of fatty acids, hydroformylation of terminal olefins, partial oxidation of n-paraffins and the Al-catalyzed polymerization of ethylene. Fatty alcohols can also be made by chemical hydration of alpha-olefins produced from petrochemical feedstocks. Unfortunately, it is not commercially viable to produce fatty alcohols directly from the oxidation of petroleum-based linear hydrocarbons (n-paraffins). This impracticality is because the oxidation of n-paraffins produces primarily secondary alcohols, tertiary alcohols or ketones, or a mixture of these compounds, but does not produce high yields of fatty alcohols. Additionally, currently known methods for producing fatty alcohols suffer from the disadvantage that they are restricted to feedstock which is relatively expensive, notably ethylene, which is produced via the thermal cracking of petroleum. In addition, current methods require several steps, and several catalyst types.
Plant primary fatty alcohols occur either in free form or are linked by an ester-bond with a fatty acid, e.g. palmitic acid, to give a wax ester or an aromatic compound, e.g. ferulic acid, to give an alkyl hydroxycinnamate. These various compounds are often components of plant extracellular lipid barriers: cuticle coating the aerial surfaces, suberin found in the cell walls of various internal and external tissue layers, and sporopollenin found in the outer walls of pollen grains. These waxes are usually complex mixtures of very-long-chain (C20-C34) fatty acids and derivatives including primary fatty alcohols and wax esters. Wax esters can also serve as energy storage, such as in the case of jojoba (Simmondsia chinensis) seed oil.
Unlike most other plants, the oil of jojoba seeds, which constitutes between 45-55%, by weight, of the seeds, is mainly composed of very long chain monoesters of fatty acids and alcohols (97-98%, by weight) rather than triglycerides. These esters, which are commonly referred to as wax esters, are straight chain esters predominantly 36-46 carbons in length, with an ester bond approximately in the center of the chain. The oil, which exists as a liquid at room temperature, is used extensively as a raw material in the cosmetic and pharmaceutical industries for its dermatological properties. Jojoba oil is also used as an alternative to sperm oil as a lubricant and as a plasticizer. Because it is not subject to lipase hydrolysis and is thus poorly digested, jojoba oil has also been investigated as a non-caloric fat replacement in foods.
However, the relatively short supply of jojoba oil and its extremely desirable properties have resulted in a rather high price, preventing its use for commercial preparation of a large number of useful derivatives and products.
Thus, there exists a need for alternative means for cost effectively producing commercial and scalable quantities of very long chain length fatty acid derived products, including jojoba oil.
Previously, synthesis of long chain fatty alcohols and very long chain wax ester have only been demonstrated in yeast and Escherichia coli when heterologous expression of particular enzymes, including fatty acid reductase (FAR) and wax ester synthases, is combined with feeding of fatty acid substrates or relevant precursors (Kalscheuer et al., 2006; Li et al., 2008; Teerawanichpan and Qiu, 2010). However, these solutions are not suitable for producing scalable quantities of very long chain fatty acid derived products in a cost-effective manner.